The Mantoux tuberculin skin test is the standard method of determining whether a person is infected with Mycobacterium tuberculosis or have exposure to tubercle bacilli. It was developed by Koch in 1890 but the intradermal technique currently in use was described in 1912 by a French physician, Charles Mantoux, after whom the test is named.

Administration of the test

Tuberculin skin test (TST) is an intradermal injection. It is performed by injecting 0.1 ml of tuberculin purified protein derivative (PPD) into the inner surface of the forearm. The injection should be made with a tuberculin syringe, with the needle bevel facing upward. When placed correctly, the injection should produce a pale elevation of the skin (a wheal) 6 to 10 mm in diameter.

Principle:

Tuberculin skin test is the classic clinical demonstration of the function of the delayed-type hypersensitivity response . When a antigen i.e protein purified derivative (PPD) of tubercle bacilli is injected intradermally in an individual , immune response of person who has been exposed to the bacteria is expected to mount within 48-72 hrs leading to the formation of induration (a raised bump in the area of injection) which is due to the influx and activation of macrophages.

Test Procedure :

Allow the person to be tested to sit in a comfortable chair and place his hands on a table turning the inner side of forearm up.

Clean the test area and allowed it to air dry.

Inject a standard dose of five tuberculin units (TU) (0.1ml) intradermally (into the skin) using 28 or 26-gauge needle or tuberculin syringe from which 0.1 ml can be delivered accurately. (The fluid makes a little bump (wheal) under the skin. A circle may be drawn around the test area with a pen.

Results should be read 48 to 72 h later.

Reading the results:

The skin test reaction should be read between 48 and 72 hours after administration. The reaction should be measured in millimeters of the induration (palpable, raised, hardened area or swelling). The reader should not measure erythema (redness). The diameter of the indurated area should be measured across the forearm (perpendicular to the long axis)

If a person does not return within 48-72 hours for a tuberculin skin test reading, a second test can be placed as soon as possible. There is no contraindication to repeating the TST, unless a previous TST was associated with a severe reaction.

Interpretation of Tuberculin skin test (montoux test) results

Interpretation of tuberculin skin test always remains difficult and controversial. Various factors like age, immunological status coexisting illness etc influence its outcome, so also its interpretation. Utmost care is required while interpreting the result and giving an opinion.

According to Center for Disease Control (CDC), skin test interpretation depends on two factors:

Measurement in millimeters of the induration

Person’s risk of being infected with TB and of progression to disease if infected

Persons who are immunosuppressed for other reasons (e.g., taking the equivalent of >15 mg/day of prednisone for 1 month or longer, taking TNF-a antagonists)

An induration of 10 or more millimeters is considered positive in

Recent immigrants (< 5 years) from high-prevalence countries

Injection drug users

Residents and employees of high-risk congregate settings

Mycobacteriology laboratory personnel

Persons with clinical conditions that place them at high risk

Children < 4 years of age

Infants, children, and adolescents exposed to adults in high-risk categories

An induration of 15 or more millimeters is considered positive in

any person, including persons with no known risk factors for TB. However, targeted skin testing programs should only be conducted among high-risk groups.

Limitations of the test:

Although widely used the test has several limitations

A positive reaction may be observed in both latent and active TB infection ,therefore,it is unreliable in differentiating whether the person is currently having TB or had been infected in past or at carrier stage. Confirmatory tests such as chest X-ray, sputum culture, or both—are usually done to rule out an active TB infection.

False-Positive Reactions
Some persons may react to the tuberculin skin test even though they are not infected with M. tuberculosis. The causes of these false-positive reactions may include, but are not limited to, the following:

Infection with nontuberculosis mycobacteria

Previous BCG vaccination

Incorrect method of TST administration

Incorrect interpretation of reaction

Incorrect bottle of antigen used

False-Negative Reactions

A negative Mantoux test result usually signifies that the individual has never been exposed to M. tuberculosis. However, there are factors that may cause a false-negative result or diminished ability to respond to tuberculin even if the persons are infected with M. tuberculosis. The reasons for these false-negative reactions may include, but are not limited to, the following:

Cutaneous anergy (anergy is the inability to react to skin tests because of a weakened immune system)

Recent TB infection (within 8-10 weeks of exposure)

Very old TB infection (many years)

Very young age (less than 6 months old)

Recent live-virus vaccination (e.g., measles and smallpox)

Overwhelming TB disease

Some viral illnesses (e.g., measles and chicken pox)

Incorrect method of TST administration

Incorrect interpretation of reaction

Who Can Receive a tuberculin skin test?

Most persons can receive a TST. TST is contraindicated only for persons who have had a severe reaction (e.g., necrosis, blistering, anaphylactic shock, or ulcerations) to a previous TST. It is not contraindicated for any other persons, including infants, children, pregnant women, persons who are HIV-infected, or persons who have been vaccinated with BCG.

Vaccination with live viruses may interfere with TST reactions. For persons scheduled to receive a TST, testing should be done as follows:

Either on the same day as vaccination with live-virus vaccine or 4-6 weeks after the administration of the live-virus vaccine

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I am working as Microbiologist in National Public Health Laboratory (NPHL), government national reference laboratory under the Department of health services (DoHS), Nepal. Key areas of my work lies in Bacteriology, especially in Antimicrobial resistance.